The present invention concerns a method for measuring the blood pressure of a person by using an arm band mounted on a limb of the person, and having an inflatable cuff containing a gas under pressure.
The invention also concerns a sphygmomanometer for performing this method.
According to known methods the blood pressure is measured on the following manner:
the arm band, with the cuff connected to a pump, is fixed to the arm of the person;
then the pressure of gas in thee cuff is increased to a starting pressure which is greater than the probable systolic pressure of the person;
then the gas pressure in the cuff is decreased from the starting pressure to a pressure lower than the diastolic pressure of the person;
during this decrease in pressure, the sounds emitted in a zone comprising the arm band are sensed and those sounds, whose intensity is greater than a certain threshold level are validated, and
the pressure in the cuff is measured when the first and the last validation of a sound occur. The pressure measured at the instant of the first validation is the systolic pressure and the pressure measured at the instant of the last validation is the diastolic pressure.
The U.S. Pat. No. 3,450,131 for example discloses a sphygmomanometer comprising a microphone for detecting the so-called Korotkoff sounds emitted by the artery of the person under examination. The microphone is connected via an amplifier to the inputs of three pass band filters, whose individual outputs are connected to a respective Schmitt trigger. These triggers produce rectangular pulses during the time intervals when the received signals are greater than a predetermined threshold. These pulses are applied to a logic circuit in order to control the recording of the pressure.
The cuff of the arm band is inflated to a pressure greater than the systolic pressure expected for the person, then the pressure in the cuff is slowly decreased.
The Korotkoff sounds produced at certain pressures are converted to electric signals by the microphone. The logic circuit at the output of the Schmitt triggers is designed so that signals having a 1000 Hz component are identified as parasite sounds, whilst signals having 40 Hz and 100 Hz components but no 1000 Hz component are identified as Korotkoff sounds. For each signal identified as a Korotkoff sound, the pressure in the cuff is measured by a pressure sensor and is recorded by a pressure recorder. The first value recorded then corresponds to the systolic pressure and the last value recorded corresponds to the diastolic pressure.
Thus in this known sphygmomanometer, the measurement of systolic and diastolic pressures is made in a manner independent of the characteristics of an individual person, and particularly in a manner independent of the absolute value of these pressures. If the Schmitt triggers are set to threshold values corresponding to average Korotkoff sound levels obtained through the evaluation of a large number of persons, it is possible that the Korotkoff sounds will be so weak that amplitude of the signals transmitted to the Schmitt triggers will not attain the said threshold levels. This leads to considerable measurement errors or may even render the measurement of blood pressure impossible. If on the other hand, Schmitt triggers are set to sufficiently low thresholds to detect extremely weak Korotkoff sounds, the distinction between the real Korotkoff sounds and the parasitic sounds will be very difficult in the case where the range of Korotkoff sounds has a medium or great intensity; this will cause numerous measurement errors.